Correctly handle generic functions (e.g.: `Object.freeze`) passed as callbacks to generic functions (e.g.: `Array.prototype.map`) #42862

ExE-Boss · 2021-02-18T18:33:45Z

Suggestion

🔍 Search Terms

array map generic callback
object.freeze array map

✅ Viability Checklist

My suggestion meets these guidelines:

This wouldn't be a breaking change in existing TypeScript/JavaScript code
This wouldn't change the runtime behavior of existing JavaScript code
This could be implemented without emitting different JS based on the types of the expressions
This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

A generic transforming function (e.g.: Object.freeze) passed as a callback to a generic mapping function (such as Array.prototype.map), should be handled similarly to an arrow function with inferred types.

📃 Motivating Example

From engine262/src/engine.mjs:

// @target: ES2015
// @filename: index.ts
export const FEATURES = Object.freeze([
//                 ^?
	{
		name: 'Top-Level Await',
		flag: 'top-level-await',
		url: 'https://github.com/tc39/proposal-top-level-await',
	},
	{
		name: 'Hashbang Grammar',
		flag: 'hashbang',
		url: 'https://github.com/tc39/proposal-hashbang',
	},
	{
		name: 'RegExp Match Indices',
		flag: 'regexp-match-indices',
		url: 'https://github.com/tc39/proposal-regexp-match-indices',
	},
	{
		name: 'FinalizationRegistry.prototype.cleanupSome',
		flag: 'cleanup-some',
		url: 'https://github.com/tc39/proposal-cleanup-some',
	},
	{
		name: 'Arbitrary Module Namespace Names',
		flag: 'arbitrary-module-namespace-names',
		url: 'https://github.com/tc39/ecma262/pull/2154',
	},
	{
		name: 'At Method',
		flag: 'at-method',
		url: 'https://github.com/tc39/proposal-item-method',
	},
].map(Object.freeze));

Workbench Repro

Expected type:

export const FEATURES: readonly {
	readonly name: string;
	readonly flag: string;
	readonly url: string;
}[];

Like when using an arrow function:

// @target: ES2015
// @filename: index.ts
export const ARROW_FEATURES = Object.freeze([
//                       ^?
	{
		name: 'Top-Level Await',
		flag: 'top-level-await',
		url: 'https://github.com/tc39/proposal-top-level-await',
	},
	{
		name: 'Hashbang Grammar',
		flag: 'hashbang',
		url: 'https://github.com/tc39/proposal-hashbang',
	},
	{
		name: 'RegExp Match Indices',
		flag: 'regexp-match-indices',
		url: 'https://github.com/tc39/proposal-regexp-match-indices',
	},
	{
		name: 'FinalizationRegistry.prototype.cleanupSome',
		flag: 'cleanup-some',
		url: 'https://github.com/tc39/proposal-cleanup-some',
	},
	{
		name: 'Arbitrary Module Namespace Names',
		flag: 'arbitrary-module-namespace-names',
		url: 'https://github.com/tc39/ecma262/pull/2154',
	},
	{
		name: 'At Method',
		flag: 'at-method',
		url: 'https://github.com/tc39/proposal-item-method',
	},
].map((feature) => Object.freeze(feature)));

Workbench Repro

Actual type:

export const FEATURES: readonly Readonly<unknown>[];

💻 Use Cases

Getting correct type inference for JavaScript code on the wild web.

Related issues

Support readonly type operator to allow correct definition of Object.freeze #10725

The text was updated successfully, but these errors were encountered:

jsmart523 · 2021-09-09T14:17:34Z

Another example:

type Widget = {name:string};

const widgets: Widget[] = [
  {name: 'gear'},
  {name: 'bearing'},
  {name: 'axle'}
];

const workingReadonlyWidgets = Readonly<Widget>[] = widgets.map(
  w => Object.freeze(w)
);

const brokenReadonlyWidgets = Readonly<Widget>[] = widgets.map(
  Object.freeze
); // this line complains

typescript-bot · 2022-04-13T23:30:00Z

👋 Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of the repro in the issue body running against the nightly TypeScript.

Issue body code block by @ExE-Boss

⚠️ Assertions:

const FEATURES: readonly Readonly[]

Historical Information

Version	Reproduction Outputs
4.2.2, 4.3.2, 4.4.2, 4.5.2, 4.6.2	⚠️ Assertions: `const FEATURES: readonly Readonly[]`

RyanCavanaugh added In Discussion Not yet reached consensus Suggestion An idea for TypeScript labels Feb 18, 2021

typescript-bot added the Has Repro This issue has compiler-backed repros: https://aka.ms/ts-repros label Feb 19, 2021

ExE-Boss mentioned this issue Jul 5, 2022

Support readonly type operator to allow correct definition of Object.freeze #10725

Closed

Correctly handle generic functions (e.g.: `Object.freeze`) passed as callbacks to generic functions (e.g.: `Array.prototype.map`) #42862

Correctly handle generic functions (e.g.: `Object.freeze`) passed as callbacks to generic functions (e.g.: `Array.prototype.map`) #42862

ExE-Boss commented Feb 18, 2021 •

edited

jsmart523 commented Sep 9, 2021

typescript-bot commented Apr 13, 2022 •

edited

Correctly handle generic functions (e.g.: Object.freeze) passed as callbacks to generic functions (e.g.: Array.prototype.map) #42862

Correctly handle generic functions (e.g.: Object.freeze) passed as callbacks to generic functions (e.g.: Array.prototype.map) #42862

Comments

ExE-Boss commented Feb 18, 2021 • edited

Suggestion

🔍 Search Terms

✅ Viability Checklist

⭐ Suggestion

📃 Motivating Example

Expected type:

Actual type:

💻 Use Cases

Related issues

jsmart523 commented Sep 9, 2021

typescript-bot commented Apr 13, 2022 • edited

Correctly handle generic functions (e.g.: `Object.freeze`) passed as callbacks to generic functions (e.g.: `Array.prototype.map`) #42862

Correctly handle generic functions (e.g.: `Object.freeze`) passed as callbacks to generic functions (e.g.: `Array.prototype.map`) #42862

ExE-Boss commented Feb 18, 2021 •

edited

typescript-bot commented Apr 13, 2022 •

edited